Personally, I do not know anyone that has left engineering for any reason, other than maternity leave or because it was the first step in launching their career. The article mentions stepping stone.
It takes a lot of time and effort, both in school, and on the job, to simply move onto something else. Granted, sometimes things pop up in life that may come into play, but to willingly leave engineering is something I cannot grasp. Sure, you may hate it, but so do most people I work with. Doesn't mean you up and leave a chosen career. I have thought about it, but it was always to advance myself and use my engineering background a a stepping stone.
I guess it is possible, but the numbers in the article are a little far fetched imo. I have to agree that maybe they were never an engineer to begin with, hit by a poor economy, or simply moved into management and they are possible not included.